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| <h3 class="section">5.3 Symbol Names</h3> |
| |
| <p><a name="index-symbol-names-213"></a><a name="index-names_002c-symbol-214"></a>Symbol names begin with a letter or with one of ‘<samp><span class="samp">._</span></samp>’. On most |
| machines, you can also use <code>$</code> in symbol names; exceptions are |
| noted in <a href="Machine-Dependencies.html#Machine-Dependencies">Machine Dependencies</a>. That character may be followed by any |
| string of digits, letters, dollar signs (unless otherwise noted for a |
| particular target machine), and underscores. |
| |
| <p>Case of letters is significant: <code>foo</code> is a different symbol name |
| than <code>Foo</code>. |
| |
| <p>Each symbol has exactly one name. Each name in an assembly language program |
| refers to exactly one symbol. You may use that symbol name any number of times |
| in a program. |
| |
| <h4 class="subheading">Local Symbol Names</h4> |
| |
| <p><a name="index-local-symbol-names-215"></a><a name="index-symbol-names_002c-local-216"></a>A local symbol is any symbol beginning with certain local label prefixes. |
| By default, the local label prefix is ‘<samp><span class="samp">.L</span></samp>’ for ELF systems or |
| ‘<samp><span class="samp">L</span></samp>’ for traditional a.out systems, but each target may have its own |
| set of local label prefixes. |
| On the HPPA local symbols begin with ‘<samp><span class="samp">L$</span></samp>’. |
| |
| <p>Local symbols are defined and used within the assembler, but they are |
| normally not saved in object files. Thus, they are not visible when debugging. |
| You may use the ‘<samp><span class="samp">-L</span></samp>’ option (see <a href="L.html#L">Include Local Symbols: <samp><span class="option">-L</span></samp></a>) to retain the local symbols in the object files. |
| |
| <h4 class="subheading">Local Labels</h4> |
| |
| <p><a name="index-local-labels-217"></a><a name="index-temporary-symbol-names-218"></a><a name="index-symbol-names_002c-temporary-219"></a>Local labels help compilers and programmers use names temporarily. |
| They create symbols which are guaranteed to be unique over the entire scope of |
| the input source code and which can be referred to by a simple notation. |
| To define a local label, write a label of the form ‘<samp><b>N</b><span class="samp">:</span></samp>’ (where <b>N</b> |
| represents any positive integer). To refer to the most recent previous |
| definition of that label write ‘<samp><b>N</b><span class="samp">b</span></samp>’, using the same number as when |
| you defined the label. To refer to the next definition of a local label, write |
| ‘<samp><b>N</b><span class="samp">f</span></samp>’—the ‘<samp><span class="samp">b</span></samp>’ stands for “backwards” and the ‘<samp><span class="samp">f</span></samp>’ stands |
| for “forwards”. |
| |
| <p>There is no restriction on how you can use these labels, and you can reuse them |
| too. So that it is possible to repeatedly define the same local label (using |
| the same number ‘<samp><b>N</b></samp>’), although you can only refer to the most recently |
| defined local label of that number (for a backwards reference) or the next |
| definition of a specific local label for a forward reference. It is also worth |
| noting that the first 10 local labels (‘<samp><b>0:</b></samp>’<small class="dots">...</small>‘<samp><b>9:</b></samp>’) are |
| implemented in a slightly more efficient manner than the others. |
| |
| <p>Here is an example: |
| |
| <pre class="smallexample"> 1: branch 1f |
| 2: branch 1b |
| 1: branch 2f |
| 2: branch 1b |
| </pre> |
| <p>Which is the equivalent of: |
| |
| <pre class="smallexample"> label_1: branch label_3 |
| label_2: branch label_1 |
| label_3: branch label_4 |
| label_4: branch label_3 |
| </pre> |
| <p>Local label names are only a notational device. They are immediately |
| transformed into more conventional symbol names before the assembler uses them. |
| The symbol names are stored in the symbol table, appear in error messages, and |
| are optionally emitted to the object file. The names are constructed using |
| these parts: |
| |
| <dl> |
| <dt><em>local label prefix</em><dd>All local symbols begin with the system-specific local label prefix. |
| Normally both <samp><span class="command">as</span></samp> and <code>ld</code> forget symbols |
| that start with the local label prefix. These labels are |
| used for symbols you are never intended to see. If you use the |
| ‘<samp><span class="samp">-L</span></samp>’ option then <samp><span class="command">as</span></samp> retains these symbols in the |
| object file. If you also instruct <code>ld</code> to retain these symbols, |
| you may use them in debugging. |
| |
| <br><dt><var>number</var><dd>This is the number that was used in the local label definition. So if the |
| label is written ‘<samp><span class="samp">55:</span></samp>’ then the number is ‘<samp><span class="samp">55</span></samp>’. |
| |
| <br><dt><kbd>C-B</kbd><dd>This unusual character is included so you do not accidentally invent a symbol |
| of the same name. The character has ASCII value of ‘<samp><span class="samp">\002</span></samp>’ (control-B). |
| |
| <br><dt><em>ordinal number</em><dd>This is a serial number to keep the labels distinct. The first definition of |
| ‘<samp><span class="samp">0:</span></samp>’ gets the number ‘<samp><span class="samp">1</span></samp>’. The 15th definition of ‘<samp><span class="samp">0:</span></samp>’ gets the |
| number ‘<samp><span class="samp">15</span></samp>’, and so on. Likewise the first definition of ‘<samp><span class="samp">1:</span></samp>’ gets |
| the number ‘<samp><span class="samp">1</span></samp>’ and its 15th definition gets ‘<samp><span class="samp">15</span></samp>’ as well. |
| </dl> |
| |
| <p>So for example, the first <code>1:</code> may be named <code>.L1</code><kbd>C-B</kbd><code>1</code>, and |
| the 44th <code>3:</code> may be named <code>.L3</code><kbd>C-B</kbd><code>44</code>. |
| |
| <h4 class="subheading">Dollar Local Labels</h4> |
| |
| <p><a name="index-dollar-local-symbols-220"></a> |
| <code>as</code> also supports an even more local form of local labels called |
| dollar labels. These labels go out of scope (i.e., they become undefined) as |
| soon as a non-local label is defined. Thus they remain valid for only a small |
| region of the input source code. Normal local labels, by contrast, remain in |
| scope for the entire file, or until they are redefined by another occurrence of |
| the same local label. |
| |
| <p>Dollar labels are defined in exactly the same way as ordinary local labels, |
| except that they have a dollar sign suffix to their numeric value, e.g., |
| ‘<samp><b>55$:</b></samp>’. |
| |
| <p>They can also be distinguished from ordinary local labels by their transformed |
| names which use ASCII character ‘<samp><span class="samp">\001</span></samp>’ (control-A) as the magic character |
| to distinguish them from ordinary labels. For example, the fifth definition of |
| ‘<samp><span class="samp">6$</span></samp>’ may be named ‘<samp><span class="samp">.L6</span><kbd>C-A</kbd><span class="samp">5</span></samp>’. |
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